The design goal was a low-cost (smaler chip) design, realized
by simplifying the decoder stage. There were no instructions
with the value xxxxxx11, reducing the 1-of-4 decoder to a
single NAND gate. Instructions with the value xxxxxx11
actually executed two instructions in paralell, some of them
useful.

The 6501 was pin-compatible with the 6800 for easier market
penetration. The 650x-series had an on-chip clock oscillator
while the 651x-series had none.

The PET was an idea of the of the 6500 developers. It was
completly developed by MOS, but was manufactured and marketed
by Commodore. By the time the it was ready for production
(and Commodore had cancelled all orders) MOS had been taken
over by Rockwell (Commodore's parent company). Just at this
time the 6522 (VIA) was finished, but the data sheet for it
was not and its developers had left MOS. For years, Rockwell
didn't know in detail how the VIA worked.